This invention generally relates to an X-ray radiation (or dose) control apparatus, and more particularly to an X-ray radiation apparatus which can stabilize X-ray radiation (or dose) from an X-ray tube by filtering the ripple components from the high voltage applied between the anode and the cathode of the X-ray tube.
X-ray radiation stability is one of the most significant factors for an X-ray diagnostic apparatus, because it largely determines the accuracy of the diagnostic information obtained thereby. In order to stabilize X-ray radiation a stable voltage is required to be applied between the anode and the cathode of the X-ray tube, and the heating voltage for the filament of the X-ray tube has to be kept constant.
A method of stabilizing the voltage to be applied to the X-ray tube is known. This method consists of using an electron tube such as a tetrode connected between a high voltage source and an X-ray tube. The tetrode can remove ripple components due to its internal voltage drop from a high DC voltage produced by raising the voltage of a three-phase power source and rectifying it in the full-wave rectification, thereby applying a substantially perfect DC voltage between the anode and the cathode of the X-ray tube. The principle of the method is based on the dynamic plate (anode) resistance which plays an important part in the calculation of a tube amplifier gain, where the tube is often treated as a variable resistor.
Japanese patent application No. 53-9773 filed by Tokyo Shibaura Denki K.K., Japan [Japanese Unexamined (Kokai) patent application No. 54-102994] discloses a method of controlling the voltages to be applied to an X-ray tube by utilizing the internal voltage drop of a tetrode which is series-connected to the anode and the cathode of the X-ray tube.
A typically known X-ray control circuit based on the above-mentioned principle is shown in FIG. 1. The circuit comprises a circuit breaker 1, a high voltage transformer 2, and a full-wave rectifier bridge circuit 3. The circuit breaker 1 is connected between a three-phase power source (not shown) and the primary winding of the Delta connected high voltage transformer 2. The secondary windings of the transformer 2, are Delta-and Star-connected to the input of the full-wave rectifier bridge circuit 3. A first tetrode 4 is connected between the anode of an X-ray tube 6 and the output of the rectifier bridge circuit 3, and a second tetrode 5 is connected between the cathode of the X-ray tube 5 and the output of the rectifier bridge circuit 3, whereby the DC voltages obtained from the bridge circuit 3 are lowered by the tetrodes 4 and 5 to predetermined supply voltages (hereinafter called "an X-ray tube supply voltage"). This is accomplished by means of the anode voltage drops of the tetrodes 4 and 5 so as to generate X-rays for diagnostic purposes. The X-ray control circuit further comprises two tetrode control circuits 7 and 8 which are connected to the anode and the cathode of the X-ray tube 6 for controlling the tetrodes 4 and 5, respectively. More specifically, either tetrode control circuit changes the grid bias voltage of the corresponding tetrode, to thereby vary the anode voltage drop of the tetrode. Two high-tension bleeder resistors 10 and 11 are connected at one end to each other (ground) and at the other end to the anode and the cathode of the X-ray tube 6, respectively. A comparator amplifier 12 is connected to receive a preset tetrode control signal 9 at one input terminal and a voltage signal developed by the resistor 10 at the other terminal. A comparator amplifier 13 is also connected to receive the same preset tetrode control signal at one input terminal and a voltage signal developed by the resistor 11 at the other terminal. Each comparator amplifier compares the control signal 9 with the voltage signal and then amplifies the resultant signal. An output signal from the comparator amplifier 12 is supplied to the tetrode control circuit 7 through a high-tension isolation circuit 15. Similarly, an output signal from the comparator amplifier 13 is supplied to the tetrode control circuit 8 through a high-tension isolation circuit 14. A stabilized high-tension voltage can thus be applied to the X-ray tube.
The operation of the tetrode control circuit just described may be summarized as follows. In order to obtain the preset X-ray tube apply voltage, it is necessary to adjust the above-mentioned anode voltage drop of the tetrodes 4 and 5. To this end, the preset tetrode control signal 9 and the voltage signal developed by the bleeder resistors either 10 or 11, are supplied to the respective comparator amplifier 12 or 13 so as to produce the tetrode control signal for the tetrode control circuit 7 or 8 respectively. As a result, the. higher DC voltage applied to the X-ray tube can be stabilized to the preset X-ray tube supply voltage.
Utilizing the anode voltage drops of the tetrodes 4 and 5, each tetrode control circuit is used not only to control the high-tension voltage to be applied to the X-ray tube 6, but also to remove ripple components from the three-phase DC voltage. It must therefore have an extremely high response. The response to the tetrode control circuit directly depends on the frequency characteristics of the semiconductor voltage controlling elements e.g. transistors used in this circuit.
Generally, in order to stabilize the X-ray tube supply voltage, the required voltage variation of the tetrode anode voltage drop is more than 500 V. On the other hand, in order to filter the ripple components contained in the X-ray tube supply voltage, the above-mentioned required variation is on the order of only several tens of volts, but its required response characteristic is considerably higher than that of the first-mentioned case.
Since generally the frequency characteristics of the commercially available high-voltage withstanding transistors present deterioration in the higher frequency range i.e., f.sub.T being a low value, such a transistor can not respond to the higher frequency variation of a signal sufficiently.